It is generally known that articles, shaped in the most varied ways, such as slates, flat cladding sheets or corrugated roofing sheets, purlins, pipes or other shaped articles can be produced from aqueous suspensions of mixtures comprising hydraulic binders, fillers and reinforcing fibres.
Amongst the conventional construction materials, fibre-reinforced cement articles manufactured using asbestos and cement have already been known for decades. In the asbestos cement industry, processes based on the winding process of L. Hatschek (Austrian Patent Specification 5,970) are still the most widespread techniques for the manufacture of construction elements. The technology of this production process is described exhaustively, for example, in the book by Harald Klos entitled Asbestzement ("Asbestos Cement"), Springer Verlag, 1967. Other processes applicable are for example Magnani, Mazza, Flow-on, extrusion and injection.
The Hatschek process for the manufacture of, for example, asbestos cement sheets is based on the use of cylindrical screen dewatering machines. In this process, a mat made from a dilute asbestos cement suspension contained in a vat is transferred to a felt, via a sieve cylinder, and is wound up to the desired thickness with the aid of forming drums. For the manufacture of corrugated sheets, the asbestos cement sheet formed on the forming drum is cut off from the same after the desired thickness has been reached. This sheet is then formed into shape and left to harden between oiled corrugated metal templates.
Asbestos has both, reinforcing properties linked to its intrinsic tensile strength and also process qualities related to the excellent dispersion capability in an aqueous cement suspension. During the dewatering step, because of the good filtration properties and the good cement affinity, asbestos fibres can hold back the fine particles in the suspension of the composite mixture which is being formed. In the hydrated end product, the high tensile strength combined with the high modulus of elasticity and the small elongation at break contribute to give asbestos cement articles the known high bending strength.
In the past few years, however, asbestos has become an undesirable component for reasons related to environment and health and major efforts have been invested in attempts to substitute it.
During recent years, an intensive research activity has been pursued to find substitution fibres which can partially or totally replace asbestos in existing production processes based on dewatering methods.
It is thus desirable to use new fibres as reinforcing agents and also as processing aids for use with hydraulic binders, for example for reinforcing cement. These fibres must be able to confer to fibre-containing articles the desired mechanical properties, which were formerly obtained with asbestos.
The requirements to be satisfied by fibres which are suitable for reinforcing cement and other hydraulically setting binders are exceptionnally high.
The following properties characterize asbestos both as reinforcing and processing fibre in the dewatering technology:
1) regarding processing qualities: PA0 2) regarding reinforcing qualities: PA0 (a) Measuring machine : ALC/GPC TYPE 150C, Waters Laboratory Co. PA0 (b) Column: TSK-GER GMH6-HT (high temperature type) PA0 (c) Solvent: orthodichlorobenzene (ODCB) PA0 (d) Temperature: 135.degree. C. PA0 (e) Detector: differential thermal refractometer PA0 (f) Volume of flowing solvent: 1 ml/min. PA0 Mw: the weight-average molecular weight, PA0 Mw=[.SIGMA.NiMi.sup.2 ]/[.SIGMA.NiMi ]; PA0 Mn: the number-average molecular weight, PA0 Mn=[.SIGMA.NiMi]/[.SIGMA.Ni]; PA0 Q: the ratio Mw/Mn; PA0 MFR: the melt flow range.
high specific surface PA1 good dispersing ability PA1 excellent chemical resistance and durability PA1 high cement retention capacity PA1 X good layer formation capacity PA1 high tensile strength PA1 high modulus of elasticity PA1 low elongation at break
Concerning chemical requirements, alkali resistance in saturated calcium hydroxide solutions at elevated temperature is, in particular, an absolute prerequisite.
No other natural or synthetic fibres have been found that exhibit the combination of all the properties of asbestos fibres. It is now known that the replacement of asbestos requires two distinct types of fibres corresponding to the two main functions of asbestos (see for example DE 3.002.484). The filtration properties of asbestos can be reproduced through additions of natural or synthetic pulps for example cellulose alone and/or synthetic fibrids. Selected reinforcing fibres are used for the composite reinforcement. These may be organic or inorganic high modulus fibres which are usually cut in lengths of from 1 to 15 mm.
Many synthetic fibres have been tested for the reinforcement of cement; unfortunately, most have given poor or unsatisfactory results for a number of reasons like insufficient chemical resistance, poor cement affinity, insufficient mechanical properties in particular the insufficient intrinsic tenacity and elastic modulus and an excessive elongation at break. The high price is very often a limiting factor for industrial applications.
Furthermore, the physical characteristics of the fibres should be compatible as regards important properties, with those of the hydraulic binders. In the case of cement, it is known that this material has a certain brittleness and, for example, can break at an elongation of about 0.03%. According to previous art, a reinforcing fibre must have a higher initial modulus than the elastic modulus of the hydraulic binder.
In addition to the abovementioned physical properties of fibres, it is likewise important that the fibres can readily be dispersed in a dilute aqueous cement suspension and also remain uniformly dispersed on adding further additives, if these fibres are to be processed by draining processes to give fibre cement products.
The literature already contains innumerable publications on the use of various natural, synthetic, organic and inorganic fibres. Fibres made of cotton, cellulose, polyamide, polyester, polyacrylonitrile, polypropylene and polyvinyl alcohol, inter alia, have already been investigated for reinforcing cement. Likewise, work with fibres made of glass, steel, aramide and carbon is known. Of all these fibres, none has hitherto had all the requirements needed, especially for cement.
For example, glass has a low chemical stability, steel shows corrosion and has a too high density, carbon is too brittle, shows low adhesion and has a high price, cellulose has insufficient durability, polyethylene and standard polypropylene have insufficient tensile strength.
To date, there are mainly two types of synthetic fibres which satisfy the requirements for the reinforcement of cement. Both are high modulus fibres based on polyvinyl alcohol (or PVA) and polyacrylonitrile (or PAN) polymers alone (GB 2.850.298) or in combination. The first is available for example, under the trade mark Kuralon.RTM. from the firm Kuraray, Japan (DE 2.850.337), an example of the second is Dolanit.RTM. produced by Hoechst, Germany.
These fibres are characterized by a high tenacity and a low elongation at break as illustrated hereinbelow.
______________________________________ PVA PAN ______________________________________ Tenacity N/mm.sup.2 1550 910 Initial modulus N/mm.sup.2 37000 17000 Elongation at break (%) 7.4 9.0 ______________________________________
In the field of fibre-cement, it is known that the mechanical strength is lowest when the composites are in the wet state (a common situation when exposed in the environment) and thus International Standards often require measurements to be done in water saturated conditions. In addition, the energy of fracture is also a very important property because it provides evidence of the impact toughness of the article.
PVA fibres with the better mechanical properties not only give a higher bending strength in the wet state but the energy of fracture is much higher than in the case of PAN fibres. The energy of fracture is defined as the area under the stress strain curve up to the point at which the maximum bending strength is reached, i.e. when the composite is ruptured.
The shortcoming of PVA fibres is their sensitivity to water at high temperature and their high price. Shaped articles reinforced by PVA show excellent mechanical properties in the dry state but the high level of bending strength decreases in wet state.
In view of the correlations between fibres data and the resulting article properties, it is relatively simple to produce fibre-cement articles upon which high standards are set as regards bending strength, impact toughness and energy of fracture using exclusively PVA as reinforcing fibres. PVA fibres are indeed very expensive (at least 50% higher than the less efficient PAN fibres). One patented solution proposed was to use selected mixtures of both PVA and PAN which give better results than expected from the law of mixtures (EP 0.155.520).
Although this solution is attractive from the economical point of view, the energy of fracture remains still at the lower level.
The object of the present invention is to provide fibre reinforced shaped solid articles which avoid the disadvantages of the prior art, e.g. low energy of fracture in the wet state and high price.
Based on the rule of mixtures for strength of a fibre-matrix composite, only high modulus and high tenacity fibres have been used so far for the manufacturing of fibre-cement articles with high bending strength.
A pure cement matrix has an E-modulus of 15000 N/mm.sup.2. Therefore, according to the rule of mixtures for a fibre-cement composite, it must be assumed that the E-modulus of the reinforcing fibre must be higher than 15000 N/mm.sup.2. This theorical assumption has also been confirmed to date by practical experience.
In this view, it has always been asserted that in general, polypropylene fibres are technically poor when it comes to reinforcing cement based materials in direct tension or flexure in the relatively brittle matrix of cements and mortars. Indeed, it was altogether improbable to get results comparable to high modulus polyvinyl alcohol (PVA) fibres which are the best asbestos substitute to date.